Nothing to do with battery water, if your alternator was not charging the battery, then got ocean inside also no use...

Spark plug problem, you would have most likely notice when you were driving before, i.e. car jerks a fair bit when you try to accelerate, cos not all cylinders were combusted properly/completely.

Alternator cannot check on your own la, you will need a mechanic to check that.

Next step i would suggest is to try a jump-start first.